The goals of designers of both commercial and military aircraft include increasing fuel efficiency and performance. One way of obtaining both of these goals is to substitute lighter materials for the metal alloys that are used throughout the aircraft industry. Parts made from composite materials incorporating resins and fibers show great promise for reducing the weight of a given aircraft component while providing strength that is equal to or greater than the same part made of metal.
Unfortunately, present techniques for manufacturing composite articles that incorporate fibers are typically labor intensive, resulting in a cost-per-part that may be prohibitive for many aircraft applications. The "lay-up" for some composite parts must be done by hand, and for those parts which are to be cured in an autoclave the procedure for setting up the composite part for curing into the desired shape may be very complicated, depending on the shape of the part to be produced.
One prior art method of curing composite parts includes curing the parts in an autoclave. Because the material used in most autoclaves to provide the hydrostatic pressure will intrude into a lay-up at the temperatures and pressures required for curing, the part must be isolated from the hydrostatic material during the curing process. Typically, the part is isolated by sealing the composite lay-up in a polymer bag such as are commercially available and known in the industry. A vacuum is then drawn on the inside of the bag to evacuate any air from the bag along with any gasses that might have been given off by the composite material. Lay-ups with complex shapes, such as where a flange meets the surface of a cylinder, are difficult to bag and seal. When a vacuum is drawn on the inside of the bag, a portion of the bag may end up bridging from the flange to the cylinder without laying up flush to the fillet where the cylinder and flange meet. As the vacuum increases, that portion of the bag must either stretch or break.
If the bag tears while the vacuum is being drawn, the lay-up must be removed from the damaged bag and sealed inside a new bag and the procedure repeated. If the bag merely stretches, it may tear at the high pressure and temperature of the autoclave curing process. If this occurs, the hydrostatic material of the autoclave intrudes into the lay-up and ruins it, thereby reducing overall yield.
To prevent the polymer bag from bridging while the vacuum is being drawn, pleats are folded into the bag at those locations where the likelihood of bridging is high. On a part having many complex features, this may require a great deal of labor in folding, adjusting, and refolding the bag to ensure that adequate material is present to allow the bag to lay flush on the part after being subjected to a vacuum, without tearing. Thus, the labor cost associated with preparing the polymer bag is significant, and requires the efforts of a skilled technician.
What is needed is a method of isolating the lay-ups of complex shapes that is less labor intensive and less susceptible to tearing during autoclave curing than the polymer bag method of the prior art, yet effectively isolates the lay-up from the hydrostatic material of the autoclave.